[Sync] Make THEMES a priority type

This CL makes THEMES data type a priority type. This gives the data type
preference in intial sync (and thus is downloaded very fast, independent
of how many regular sync entities the user has). This also gives THEMES
preference in commits.

Bug: 1084970
Change-Id: I53bf7c85c73694406ba96397da1fa8e083c8aee7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2220026
Commit-Queue: Marc Treib <treib@chromium.org>
Auto-Submit: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Cr-Commit-Position: refs/heads/master@{#779727}

components/sync/base/model_type.h

@@ -289,7 +289,7 @@ constexpr ModelTypeSet AlwaysEncryptedUserTypes() {
 constexpr ModelTypeSet PriorityUserTypes() {
   return ModelTypeSet(DEVICE_INFO, PRIORITY_PREFERENCES,
                       SUPERVISED_USER_SETTINGS, SUPERVISED_USER_WHITELISTS,
-                      OS_PRIORITY_PREFERENCES, SHARING_MESSAGE);
+                      OS_PRIORITY_PREFERENCES, SHARING_MESSAGE, THEMES);
 }
 
 // Proxy types are placeholder types for handling implicitly enabling real